$key = '20*50*30';
$arr = explode('*', $key);
print_r(Arrangement($arr));

//排列 Arrangement
function Arrangement($arr = array(), $res = '') {
  if(! is_array($arr) ) $arr = str_split($arr);
  if(empty($arr)) $array[] = $res;
  else foreach($arr AS $k => $v) {
    unset($arr[$k]);
    foreach( Arrangement($arr, trim("$res*$v", '*')) AS $t) $array[] = $t;
    $arr[$k]    = $v;
  }
  return  $array;
}

Array
(
    [0] => 20*50*30
    [1] => 20*30*50
    [2] => 50*30*20
    [3] => 50*20*30
    [4] => 30*20*50
    [5] => 30*50*20
)